The goal of this competitive renewal is to elucidate the apoptotic mechanism of tubular cell injury, a major cause of acute renal failure. During the last grant period, we and others have demonstrated the involvement of tubular cell apoptosis in ischemic and nephrotoxic renal injury. Importantly, these studies have suggested a pivotal role for mitochondrial signaling. Under the pathological condition, the outer membrane of mitochondria is permeabilized, resulting in the release of apoptogenic factors such as cytochrome c. Mitochondrial membrane permeabilization involves the pro-apoptotic Bcl-2 family proteins, Bax and Bak; however, the underlying mechanism is unclear. Our preliminary studies have now revealed a striking morphological change of mitochondria during tubular cell apoptosis. Importantly, the morphological change appears to be a determinant of mitochondrial permeabilization. We found: 1) upon apoptosis induction, filamentous mitochondria become fragmented;2) inhibition of mitochondrial fission or fragmentation prevents mitochondrial membrane permeabilization and apoptosis;3) Drp-1 and Endo-B1, two fission proteins, translocate to mitochondria during tubular cell apoptosis;4) Endo-B1 specifically interacts with Bax;5) while both Bax and Bak contribute to mitochondrial permeabilization, Bak appears to have a unique role in mitochondrial fragmentation. We hypothesize that upon apoptosis induction, Endo-B1 interacts with Bax, leading to Bax translocation to mitochondria. Meanwhile, Drp-1 is recruited to mitochondria and collaborates with Bak to induce mitochondrial fission and fragmentation. Membrane changes during mitochondrial fragmentation facilitate the formation of apoptotic pores by Bax, leading to the release of apoptogenic factors. We will test this hypothesis by three specific aims. Aim 1 will demonstrate mitochondrial fragmentation in vivo and its role in acute kidney injury. Aim 2 will determine the involvement of Endo-B1 in Bax activation and mitochondrial fragmentation during tubular cell apoptosis. Aim 3 will elucidate the regulation of mitochondrial morphological dynamics by Bak and Bax during apoptosis. The studies are expected to advance the fundamental understanding of mitochondrial injury during tubular cell apoptosis. Completion of the research may provide novel therapeutic strategies for ischemic and nephrotoxic renal failure.